Several approaches have been made to monitor and report errors in fixation, eyelid closure, and patient misalignment during visual field measurements in an effort to improve the reliability of visual field testing. Maintaining steady fixation on the central target is vital to accurate mapping of the visual field and for comparing follow-up tests over time. Similarly, complete or partial closure of the eyelids during stimulus presentation also can interfere with visual field measurement. Perhaps less obviously, imprecise translational (x, y and z) and/or rotational placement of the eye relative to the trial lens can also confound visual field test results. It is known to observe the eye under test in a video presentation (see for example U.S. Pat. No. 4,675,736 hereby incorporated by reference). This enables the operator to have a continuous view of the patient's eye position with respect to the trial lens holder to detect obvious deficiencies in the field test. Unfortunately, the operator may be either periodically absent or attending other tasks which divert his attention from the video presentation. Further, the operator cannot determine gaze direction from the video display and typically is unaware of when the actual point is presented, the only time when gaze direction is important.
One approach to gaze direction error detection is the Heijl-Krakau technique, in which a bright stimulus is intentionally projected to an assumed blind-spot of the patient's eye. If the patient's gaze is not on the fixation target, a portion of the retina capable of sight is now in the assumed location of the blind spot, and the patient will respond erroneously to the stimulus, which is recorded as a fixation loss. If the fixation losses exceed a predetermined threshold of the total trial, say 20%, the operator is notified. While useful, this method is problematic in that the anatomic blind spot is not always where predicted, which can lead to erroneous recordings of fixation losses. Additionally Heijl-Krakau monitoring increases overall testing time.
Alternatively, gaze tracking combining video measurement of locations of reflections off the corneal surface with detection of pupil center location have been incorporated into commercial devices. (See for example HFA-II Carl Zeiss Meditec Inc Dublin, Calif. and U.S. Pat. Nos. 5,220,361 and 5,491,757 hereby incorporated by reference). The gaze tracker records patient gaze error during each stimulus presentation, along with pupil position in the lateral dimension relative to the trial lens. Several stimulus presentations must be made at a given perimetric test point in order to determine the minimum brightness that can be seen at that location—the threshold. If the measured sensitivity at a test point location is normal or nearly normal, that result probably did not happen by accident. However, if the sensitivity is lower than normal, that result might have happened because of genuine loss of visual sensitivity, or because—during one or more of the stimulus presentations required to determine threshold sensitivity—the eye was not centered behind the trial lens holder (thus allowing the trial lens holder to block the patient's vision in that part of the visual field), or the head and eye were tilted around the visual axis, allowing the stimulus to be presented at the wrong retinal location, or because the eyelid was partially or completely closed, or because the patient was not looking where s/he was supposed to be looking. Typically, gaze trackers provide a graph of gaze deviation amplitude over time (gaze graph), but do not connect the fixation error with a specific test location in the visual field. Clinical gaze trackers provide only limited quantitative gaze error data and may not quantify other types of issues, e.g., partial eyelid closure or trial lens blocking parts of the field of view. Furthermore, there is no way for the doctor to retrospectively determine the cause of a depressed measurement.